Single-walled carbon nanotubes are a material with many potential industrial and research uses. Potential applications for carbon single-walled nanotubes (SWNTs) composed of carbon include such diverse areas as nanodevices, field emitters, improved capacitors, high strength composite materials, and hydrogen storage. Realizing the full potential of carbon nanotubes within these technologies, however, will require further study into the properties of carbon nanotubes. Some of the barriers to achieving this potential are due to limitations in current carbon nanotube synthesis methods.
The end product for many carbon nanotube synthesis techniques is a mixture of carbon nanotubes, amorphous carbon, and some type of metal particles used as a growth catalyst. The metal particles used for the growth catalyst are often metal nanoparticles composed of a ferromagnetic material such as iron, cobalt, or nickel. Additionally, the carbon nanotubes themselves are formed in a mixture of sizes and shapes. In particular, the desired carbon SWNTs are usually a mixture of carbon SWNTs with semiconducting-type properties and carbon SWNTs with metallic-type properties. The ratio of metallic type-nanotubes versus semiconducting-type nanotubes is controlled in part by the process used to synthesize the nanotubes.
Some previous work has focused on purifying carbon nanotube samples by removing the metal particles of the nanotube growth catalyst. Typically this is achieved by treating the nanotube sample with an acid, such as hydrochloric acid (HCl), nitric acid (HNO3), or another mineral acid, that will dissolve the metal without causing substantial harm to the nanotubes. However, during a carbon nanotube synthesis process, an amorphous carbon shell can form around some of the metal particles. This amorphous carbon layer can prevent the acid from reaching the metal, thus preventing dissolution and removal of the metal particles from the nanotube sample.
One method for overcoming this problem is to treat a nanotube sample containing amorphous carbon, metal growth catalyst particles, and carbon nanotubes with microwave energy in an atmosphere containing oxygen. Exposing the nanotube sample containing carbon nanotubes, metal growth catalyst particles, and amorphous carbon to microwave energy will result in heating of the sample. The primary source of heating in this situation is heating of the metal particles in the growth catalyst due to interaction of the metal particles with the magnetic field component of the microwave energy. As the metal particles increase in temperature, heat is transferred to the surrounding portions of the nanotube sample, including any amorphous carbon that has formed a shell around portions of the metal growth catalyst. If the sample is heated sufficiently the amorphous carbon becomes susceptible to oxidation, resulting in removal of the carbon as a gas phase product (such as CO or CO2) or sufficient cracking of the amorphous carbon layer to allow acids to dissolve the metal particles. Typically the treatment conditions are selected to raise the temperature of the nanotube sample to a temperature where the amorphous carbon is damaged or removed without causing significant damage to the carbon nanotubes.
While the above technique provides a method for removing metal particles from a carbon nanotube sample, the resulting sample will still contain a mixture of carbon nanotubes with metallic-type and semiconducting-type properties. The next desirable purification step would be to further enrich a nanotube sample in a single type of nanotubes. For example, semiconducting-type carbon nanotubes exhibit luminescent properties. As a result, semiconducting-type carbon nanotubes potentially could be used in a variety of technologies that could benefit from improved luminescent materials, such as computer displays. In carbon nanotube samples containing both metallic-type and semiconducting-type nanotubes, samples with higher relative proportions of semiconducting-type nanotubes exhibit stronger luminescence. Thus, what is needed is a method for selectively removing metallic-type carbon nanotubes without affecting the semiconducting-type nanotubes in a nanotube sample.